The Immune System: Structure, Function, and Mechanisms

The Immune System

Overview

The immune system is a complex network of cells, tissues, and molecules that protects the body from disease-causing pathogens, removes dead or damaged cells, and identifies abnormal "self" cells. It is divided into innate and adaptive responses, which work together to maintain health.

Objectives

• Differentiate between the roles and mechanisms of innate and adaptive immune responses, including key cell types and molecular mediators.

• Describe the physical, mechanical, and chemical barriers that constitute the first line of defense in the immune system.

• Identify the types and functions of leukocytes involved in immune responses, including granulocytes, monocytes, dendritic cells, and lymphocytes.

• Explain the molecular components and classes of the immune response, including chemokines, opsonins, pyrogens, and antibodies.

• Describe the structure and function of antibodies, including their classification (IgG, IgA, IgE, IgM, IgD) and the roles of MHC-I and MHC-II in antigen presentation.

• Discuss the process of antigen presentation and the roles in humoral immunity.

• Compare the mechanisms of active and passive immunity, including the significance of natural and artificial acquisition.

• Highlight the immune response to bacterial and viral infections, and the distinction between innate and adaptive immunity.

Major Functions of the Immune System

Three Major Functions

1. Recognition and Removal of Abnormal "Self" Cells: The immune system identifies and eliminates cells that display abnormal markers, such as cancerous or infected cells.

2. Removal of Dead or Damaged Cells: Phagocytic cells clear cellular debris and apoptotic cells to maintain tissue health.

3. Protection from Pathogens: The immune system defends against bacteria, viruses, fungi, and parasites that can cause disease.

Immune System Basics

• Development of Immune Cells: Immune cells originate from hematopoietic stem cells in the bone marrow and differentiate into various cell types, including lymphocytes (B cells, T cells), monocytes, and granulocytes.

• Molecules of the Innate Immune Response: Includes cytokines, chemokines, complement proteins, and pattern recognition receptors (PRRs) that mediate rapid, non-specific defense.

• Antigen Presentation and Recognition Molecules: Major Histocompatibility Complex (MHC) molecules present antigens to T cells, enabling adaptive immune responses.

Physical, Mechanical, and Chemical Barriers

First Line of Defense

Barriers prevent pathogens from entering the body and are essential for innate immunity.

• Physical Barriers: Skin, mucous linings, and ciliated epithelium act as protective surfaces.

• Mechanical Barriers: Removal of pathogens by processes such as coughing, sneezing, and the movement of cilia.

• Chemical Barriers: Secretions like gastric acid, lysozyme in saliva and tears, and antimicrobial peptides destroy or inhibit pathogens.

Barrier Type	Location	Function
Epithelium	Skin, mucous membranes	Physical barrier to pathogens
Glandular Secretions	Salivary glands, stomach, tears	Enzymes (e.g., lysozyme) destroy bacteria
Stomach Acidity	Stomach	Low pH kills ingested microbes
Mechanical Removal	Respiratory tract, GI tract	Cilia and fluid flow remove pathogens

Three Lines of Defense

Overview of Immune Responses

• Barriers: Physical and chemical defenses prevent pathogen entry.

• Innate Immune Response: Rapid, non-specific response involving phagocytic cells (macrophages, neutrophils), natural killer cells, and inflammatory mediators.

• Adaptive Immune Response: Specific, slower response involving lymphocytes (B cells and T cells), antibody production, and memory cell formation.

Defense Line	Key Components	Features
Barriers	Skin, mucous membranes, secretions	Prevent entry of pathogens
Innate Immunity	Phagocytes, NK cells, complement	Immediate, non-specific response
Adaptive Immunity	B cells, T cells, antibodies	Specific, memory-based response

Innate vs Adaptive Immune Responses

Comparison

• Innate Immunity: Present from birth, responds quickly to a broad range of pathogens, does not confer long-lasting immunity.

• Adaptive Immunity: Develops after exposure to specific antigens, slower to respond, provides long-term protection through memory cells.

Feature	Innate Immunity	Adaptive Immunity
Speed	Immediate	Delayed (days)
Specificity	Non-specific	Highly specific
Memory	None	Long-term memory
Main Cells	Phagocytes, NK cells	B cells, T cells

Key Terms and Concepts

• Pathogen: Any organism or agent that causes disease (e.g., bacteria, viruses, fungi).

• Antigen: A molecule recognized by the immune system as foreign, triggering an immune response.

• Antibody: A protein produced by B cells that binds specifically to antigens.

• Phagocyte: A cell that engulfs and digests pathogens and debris (e.g., macrophages, neutrophils).

• Natural Killer (NK) Cell: A lymphocyte that destroys infected or abnormal cells without prior sensitization.

• Cytokine: A signaling molecule that modulates immune responses.

• Complement System: A group of proteins that enhance immune responses and facilitate pathogen destruction.

Example: Immune Response to Infection

When a pathogen breaches physical barriers, innate immune cells respond rapidly by phagocytosis and inflammation. If the pathogen persists, antigen-presenting cells activate adaptive immunity, leading to the production of specific antibodies and cytotoxic T cells that eliminate the invader and establish immunological memory.

Additional info: Academic context and definitions have been expanded for clarity and completeness.